katrina gave us a sample of [that / what] unmanageable climate change could look like, when on august 29, 2005, that hurricane smashed new orleans with a ferocity that many scientists believe was fed by the warmer waters in the gulf of mexico attributable to global warming.

정답이 what인 이유와 문장 해석 좀 부탁 드립니다.
when절은 도통어디를 어떻게 봐야 하는 건지..